This invention relates to nuclear reactors and has particular relationship to reactors in which a coolant, typically water at critical temperature and pressure, is circulated through the nuclear core. The core includes the fuel and is disposed in the lower region of the reactor vessel. In addition, the reactor includes control-rod assemblies. Control-rod assemblies include control rods, supports for the control rods, drive rods for the control rods and guides for the control rods and, in some reactors, other related components. The core with its fuel assemblies is usually referred to as the lower internals of the reactor. The control-rod drives and guides are referred to as the upper internals of the reactor. The control rods are movable by the drive rods between the upper internals and recesses or thimbles in the core.
The control rods are mounted in clusters on their supports. There are rod clusters (RCC's) in which the rods have a high absorption cross-section for neutrons. These clusters are used to reduce the power or shut down the reactor and are moved between the core and their guides a number of times during the fuel cycle of a reactor. There are so-called grey control rods of substantially lower neutron absorption cross-section than the high-absorption RCC's and they serve for load follow or to moderate or control the power of the reactor. Grey rods are moved between the core and the guides many times (typically 5,600), substantially more than the high-absorption RCC's, during the fuel cycle of a reactor. There are water displacement rod clusters (WDRC'S). These rods are approximately of the same diameter as the RCC's. Their function is to displace the coolant, which may penetrate into the thimbles that do not receive control rods or grey rods. The water operates as a moderator. The coolant may be light water as in a PWR but it may also be liquid metal or heavy water. WDRC clusters are in the core during earlier operation, typically during the first 60% of the fuel cycle, and are raised into the upper internals during the remainder, typically 40% of the fuel cycle. Typically the control rods are tubular. In this application and in its claims, the expression "control-rods", unless modified by wording signifying a specific type of rod, means any or all of the above-described control rods.
Typically, the RCC's and the grey rods in their clusters are carried by cruciform supports and are movable inside of hollow curciform guides. These guides conventionally have slots through which coolant flows. The WDRC's are not so protected. A large number of these tubes are movable in rectangular or square guides which conventionally have holes through which the coolant flows. During the later part of the fuel cycle, the WDRC's are moved into and remain in perforated guides. All guides are part of the upper internals of the reactor.
The coolant flows vertically through the core and into the upper internals. The outlet nozzles of the vessel are disposed between the ends of the upper internals and they conduct the coolant horizontally. The coolant then flows generally horizontally or transversely through the control-rod assemblies in passing to the nozzles. Typically the vertical flow of the coolant through the core has a velocity of about 24 ft/sec. The flow through the nozzles in prior art apparatus has a velocity of about 50 ft/sec and the flow through some regions of the upper internals in prior art reactors may be as high as 30 or 40 ft/sec. At these velocities, the coolant causes thee vertical members and particularly the WDRC's, during the later part of the fuel cycle, to vibrate. At resonance velocities, the amplitude of vibration of the vertical members may be of the order of an inch or more. At higher velocities there is cavitation. The vertical members of the upper internals and particularly the WDRC's are subject to failure by reason of fatigue by the continuous vibrations induced by the transverse flow and by the stresses induced by cavitation.
It is an object of this invention to prevent failure of the components of the control-rod assemblies and particularly of the WDRC's by reason of the transverse flow of the coolant.